There is increasing evidence of the importance of the single-electron reduction of foreign compounds in the cell to form free radical intermediates. These intermediates can cause cellular damage by reacting directly with cellular macromolecules or byreacting directly with cellular macromolecules or by transferring an electron to molecular oxygen to form the cytotoxic superoxide radical anion. Little is known of the factors which govern the single-electron reduction of foreign compounds or of the subsequent reactivity of the free radical intermediates. The proposed study is an investigation of the influence of steric factors, solubilities, and one-electron reduction potentials upon the rate of reduction of nitro-aromatic compounds, iron-chelates and quinones, and in particular of a series of adrenochrome analogues, by three flavoproteins known to catalyze single electron reductions, NADPH-cytochrome c reductase, NADH-cytochrome b5 e reductase and NADH-dehydrogenase. The factors governing the further reaction of the free radical intermediates with cellular macromolecules and with oxygen will also be investigated. These studies will be conducted initially with isolated enzymes then with reconstituted systems and subcellular fractions. There is evidence to suggest that compounds which lead to an increase in the intracellular formation of superoxide might exhibit a selective toxicity for tumor cells because of a relative deficiency of tumor cells, and in particular their mitochondria, in superoxide dismutase and other enzymes which normally protect the cell against cytotoxic oxygen radicals. The identification of compounds which increase superoxide formation in vivo might lead to a new class of chemotherapeutic agents with an improved selectivity for tumor cells, devoid of some of the more toxic effects of the agents currently available. Preliminary evidence is presented that adrenochrome, which produces a large increase in superoxide production in vitro, possesses antitumor activity against murine leukemia P-388.